1. Field of the Invention
The present invention relates to an ion mobility spectrometer and especially an ion collecting device thereof.
2. Description of the Related Art
Referring to FIGS. 1 and 2, an ion mobility spectrometer includes an ion collecting device 50′, which includes an aperture grid 15′ for restraining the influence of ion drift movement in a drift region on ion collection, a Faraday plate 17′ for collecting ions, an insulation member 2 for fixing the Faraday plate 17′ and a shield cover 21′ connected with the insulation member 2. The ion mobility spectrometer further includes an amplifier circuit for amplifying an analog signal output from the Faraday plate 17′, the amplifier circuit coupled with the Faraday plate 17′ via a conductor 6 and having an analog signal ground. In addition, the ion mobility spectrometer further includes a drift electrode power source for supplying power to drift electrode. The power source has a grounded output terminal which forms a ground of the drift electrode. The shield cover 21′ is coupled with the analog signal ground of the amplifier circuit through a shield layer 5 of a coaxial cable, and is coupled with the ground supplying power to the drift electrode via an electric wire 7. The ions in drift region pass through the aperture grid 15′ and are collected by the Faraday plate 17′, which outputs a signal through a coaxial cable consisted of a core line 5 and a shield layer 6.
For the above configuration, the ions enter the collection region, and move to the shield cover 21′ under the action of an electrical field generated by the aperture grid 15′ and the shield cover 21′, such movement toward the shield cover 21′ results in loss of ions, and decreases collection efficiency.
In the example illustrated in FIG. 1, a planar aperture grid 15′ is employed, and the Faraday plate 17′ is disposed at a downstream side of the aperture grid 15′ in an ion drift direction. The disadvantage of such structure is that the ions will spread to the shield cover 21′, and consequently the collection efficiency is adversely affected.
In the example illustrated in FIG. 2, the aperture grid 15′ is of a spherical shape, accordingly the aperture grid 15′ causes the ions to converge, but the manufacturing cost for a spherical aperture grid is high.
Furthermore, in the above existing technology, the high voltage ground of the drift electrode power source which supplies power to the drift electrode and the analog signal ground of the amplifier circuit are jointly grounded at the shield cover 21′. As a result, noise from the high voltage ground will couple in the amplifier circuit, i.e. the front-end circuit, and influence the performance of the ion mobility spectrometer.